It is the goal of the proposed research to examine neutron energy dependent mechanisms of neoplastic transformation in human cells in vitro, with a particular emphasis on low energy neutrons. To this end we will use the HeLa x skin fibroblast human hybrid cell system which we have successfully employed for studies with UV, gamma, and fast neutron irradiation. The following specific aims are proposed: 1) Determination of the RBE (relative to Cs-137 gamma rays) for slow neutron induced neoplastic transformation of HeLa x skin fibroblast human hybrid cells, over the dose range 10 to 50 cGy. 2) Isolation and characterization at the cellular and molecular level of slow-neutron induced neoplastic HeLa x skin fibroblast human hybrid cells.